1. Field of the Invention
The present invention relates to a toner used in an image forming method such as electrophotography, electrostatic recording, magnetic recording, toner jet recording, etc.
2. Description of the Related Art
The various electrophotographic methods have been known. In general, a photoconductive material is used to form an electrostatic latent image on an electrostatic latent image bearing member (hereinafter, also referred to as “photosensitive member”) by a variety of methods, followed by developing the latent image with a toner as a developer to a visualized image, i.e., toner image. If necessary, the toner image is transferred onto a recording medium such as paper and then fixed onto the recording medium through heat or pressure application etc. to obtain a copy.
An image forming apparatus adopting such an image forming method includes a copying machine or printer, for example.
In recent years, an LED or LBP printer has got a major share of the printers on the market. Regarding its technical direction, the printer with a more high resolution is being demanded. In other words, the conventional printers with the resolution of 240 or 300 dpi are now replaced by printers with the higher resolution of 600, 800, or 1200 dpi. Accordingly, a developing process is demanded to realize a high definition for the high-resolution printers. Also, in the field of copying machines, the function thereof is advanced. Thus, digitalization thereof is being in progress. Such digital copying machines mainly adopt a method of forming the electrostatic latent, image with a laser and thus, there is a growing tendency for the copying machines to pursue the higher resolution. Further, along with an increased image quality, it is greatly required to attain a higher-speed response and a longer service life of the image forming apparatus.
In a developing method adopted for the above printers or copying machines, the toner image formed on the photosensitive member in a developing step is transferred onto the recording medium in a transfer step. At this time, a transfer residual toner remaining on the photosensitive member in an image area and a fog toner in a non-image area are cleaned in a cleaning step and stored in a waste toner container. Up to now, the cleaning step has been performed through blade cleaning, fur brush cleaning, roller cleaning, etc. From the viewpoint of apparatus structure, provision of a cleaning device therefor inevitably makes the apparatus large to inhibit downsizing of the apparatus. In addition, from an ecological point of view, a system with less waste toner is demanded for making effective use of the toner. Therefore, the toner high in transfer efficiency with less fogging is required.
From a viewpoint of downsizing a device, a one-component developing method is preferable because it does not require carrier particles such as glass beads or iron powder necessary for a two-component developing method so that a developing device itself can be small-sized and lightly-weighed. Further, the two-component developing method requires a device that detects a toner concentration and replenishes a necessary amount of the toner in order to maintain a constant toner concentration in a developer; therefore, the developing device becomes large and heavy. On the other hand, the one-component developing method does not require such devices, thus allowing a small-sized and lightweight developing device, and is preferable.
Further, space-saving, cost reduction, and lowering of power consumption resulting from a miniaturization of a copying machine or printer have become extremely important objects recently, and the miniaturization or a simplification of a device and a device with low power consumption are required for a fixing device.
On the other hand, a toner is generally produced through a pulverization process, in which a binder resin, a colorant, or the like, are melt-kneaded, uniformly dispersed, pulverized by a pulverizer, and classified by a classifier to obtain toner particles of a desired particle size. According to the pulverization process, however, the range of material selection is restricted if toner particle size reduction is intended. For example, a colorant dispersing resin must be sufficiently fragile and must be finely pulverized by an economically feasible production apparatus. As a result of providing a fragile colorant dispersing resin to meet such a requirement, when the colorant dispersing resin is actually pulverized at high-speed, it is liable to result in formation of particles of a broad particle size range. A fine particle (excessively pulverized particles) particularly forms in a relatively large proportion while a magnetic powder or a colorant is liable to detach from the resin during pulverization. Moreover, a toner of such a highly fragile material is liable to be further pulverized or powdered during its use as a developer toner in a copying machine or the like.
Further, in the pulverization process, it is difficult to completely uniformly disperse solid fine particles such a magnetic powder or a colorant into a resin, and depending on a degree of dispersion, the dispersion may become a cause of an increase of fogging and lowering of image density.
Thus, the pulverization process essentially poses a limit in production of small-size fine toner particles required for high resolution and high-quality images, as it is accompanied with significant deterioration of powder properties (particularly uniform chargeability and flowability of the toner).
In order to overcome the problems of the toner produced by the pulverization process and to meet such requirements as mentioned above, the production of a toner through a polymerization process is proposed.
A toner produced by a suspension polymerization (hereinafter referred to as “polymerization toner”) is produced by: dissolving or dispersing uniformly a polymerizable monomer, a colorant, a polymerization initiator, and if required, a crosslinking agent, a charge control agent, and other additives to prepare a monomer composition; and dispersing the monomer composition in a medium (aqueous phase, for example) containing a dispersion stabilizer using an appropriate agitator, and simultaneously conducting a polymerization reaction, to thereby obtain a toner particle of a desired particle diameter. In this process, a pulverization step is simply not included; therefore, fragility of the toner is not required, and a soft material can be used as a resin. In addition, there is an advantage that an exposure of a colorant to a particle surface is prevented, and a toner having a uniform triboelectric chargeability can be obtained. Further, a particle diameter distribution of the obtained toner is relatively sharp, so that a classification step may be omitted. When conducting the classification after the production of the polymerization toner, the toner can be obtained in a higher yield. The toner obtained by the polymerization process has a spherical shape; therefore, it excels in flowability and transferability and is advantageous for a high-quality image.
Up to now, in a fixing step where the toner is fixed onto a recording medium, a fixing roller surface of a material (such as a silicone rubber or a fluororesin) showing good releasability with respect to the toner is generally formed to prevent the toner from attaching onto the fixing roller surface, and in addition, the roller surface is coated by a thin film of a liquid showing good releasability such as a silicone oil and a fluorine oil to prevent an offset phenomenon of the toner and also fatigue of the fixing roller surface. The above method is very effective for preventing the offset phenomenon of the toner, but is accompanied with difficulties such that: the requirement of a device that supplies the offset-preventing liquid results in complication of the fixing device; and the applied oil induces peeling between the layers constituting the fixing roller and thus, shortens the life of the fixing roller.
Accordingly, based on a concept of not using such a silicone oil-supplying device but supplying an offset-preventing liquid from toner particles on heating, it has been proposed to incorporate a wax, such as low-molecular weight polyethylene or low-molecular weight polypropylene within toner particles.
It is known to incorporate a wax into toner particles as a wax. For example, Japanese Examined Patent Publication No. Sho 52-3304, and No. Sho 52-3305 and Japanese Patent Application Laid-open No. Sho 57-52574 disclose such techniques.
Further, Japanese Patent Applications Laid-open No. Hei 03-50559, No. Hei 02-79860, No. Hei 01-109359, No. Sho 62-14166, No. Sho 61-273554, No. Sho 61-94062, No. Sho 61-138259, No. Sho 60-252361, No. Sho 60-252360 and No. Sho 60-217366 disclose techniques by which a wax is incorporated into toner particles.
A wax is used for the purpose of improving anti-offset properties at the time of low-temperature fixing or high-temperature fixing of toners or improving fixability at the time of low-temperature fixing. On the other hand, a wax tends to cause lowering of anti-blocking property of a toner, lowering of developability because of a temperature rise in copying machines or printers, or lowering of developability because of a migration of the wax toward toner particle surfaces when the toner is left to stand under high-temperature and high-humidity conditions for a long term.
As a countermeasure for the above problems, toners produced by suspension polymerization are proposed. For example, according to the disclosure in Japanese Patent Application Laid-open No. Hei 05-341573, a polar component is added to a monomer composition in an aqueous dispersion medium, where components having polar groups contained in the monomer composition tend to become present at a surface layer portion which is an interface with an aqueous phase. Non-polar components hardly exist at the surface layer portions; therefore, toner particles can have core/shell structures.
As a result, the produced toner achieves both the anti-blocking property and the high-temperature anti-offset properties that conflict with each other by encapsulating the wax in toner particles, and can prevent the high-temperature offset without applying any wax such as oil to fixing rollers.
However, for the low-temperature fixing, the speed of migration of a wax at a core part of the toner having a core/shell structure to a toner surface layer upon the fixing operation is an important object.
Further, as disclosed in Japanese Patent Application Laid-open No. Hei 11-202553, a production method of the polymerization toner is proposed, including: conducting a suspension polymerization under the presence an oil-soluble polymerization initiator; and adding a reducing agent for a redox initiator to thereby combine the low-temperature fixing and anti-blocking properties.
Further, Japanese Patent Application Laid-open No. Hei 10-20548 proposes a polymerization polymer in which a formation of residual monomer or the like is suppressed and which has little odor by using a specific polymerization initiator. However, the proposed toners are not sufficient in low-temperature fixability.